1. Field of the Invention
The present invention relates to a combustion temperature sensor, and, more particularly, to a combustion temperature sensor that measures infrared energy emitted at several preselected wavelengths from a flame and/or a flame""s hot gas at a turbine inlet location and applies the energy signals to a calculation model to yield temperature. Particular utility for the present invention is in the field of gas turbine engines; although other utilities are contemplated herein.
2. Description of Related Art
Combustion gas turbine designers and users can benefit from knowledge of flame temperature to determine, for example, NOx and CO emission concentrations, flame control, and flame-off conditions. Knowledge of these parameters can be used for increased turbine efficiency, and increased turbine blade life and reliability, as well as decreased pollution. While much effort has been devoted in the past to the problem of flame temperature determination, previously developed systems have been lacking in the ability to come up quickly and reliably with accurate and useful flame temperature measurements.
One example of such a temperature measurement system is shown in Cashdollar et al., U.S. Pat. No. 4,142,417. This patent discloses an IR measuring pyrometer used to calculate both particle and gas temperature from an explosion or fire. In this system three or four IR wavelength measurements (1.57, 2.30, 3.46, and 5.19 um) are obtained to compute temperature of the particles and gas. Significantly, these wavelengths are chosen to avoid discrete emission bands of gases in the hot flame, e.g., those emission bands which correspond to the quantized energies of the vibrational and rotational states of molecules. The wavelength measurements, as provided by Cashdollar et al., are restricted to dust cloud flames which are optically xe2x80x9cthickxe2x80x9d (i.e., gas cloud flame is optically opaque at the chosen wavelength), to eliminate the need to compensate for background radiation. Thus, this system would be incapable of operating in an optically thin environment, such a turbine, since background radiation from the wall on the other side of the flame would be detected and would destroy any measurement obtained.
In still other prior art examples, temperature measurement is determined by detecting UV radical (e.g., OH, CO, CH, CHO, C, etc.) emission bands in the combustion chamber. For example, German Laid Open Patent Application DE 4028922/A1 and published PCT Application WO 98/07013, each disclose methodologies for temperature determination in a combustion chamber using UV spectral emissions from a variety of gaseous radicals. Radicals, by their very nature, are short-lived as compared to molecular gas constituents, and thus, determination of temperature from molecular gas components is more stable. While UV-spectral combustion temperature determination may be adequate for some purposes, such a system cannot be used for temperature control at the turbine inlet location. In addition, UV combustion temperature determination cannot provide information that can be used to improve turbine blade life and stability.
It has been proposed (e.g., En Urga Paper 1997) to determine temperature by observing the entire IR spectrum and directly correlating certain radiation intensities of molecular CO2 and H2O. However, the harsh operating environment inside the turbine prohibit such a direct measurement. In addition the cost of producing a fiber optic fiber that is capable of both transmitting the entire spectrum without degrading in the harsh operating environment is too prohibitive. Thus, engineering trade-offs must be reached between the ability to effectively observe and transmit optical energy signals within a turbine environment, and to obtain appropriate IR wavelength intensities for accurate temperature measurement.
The present invention solves the aforementioned shortcomings of the prior art by selecting an optical fiber and detector that can withstand the operating environment of a turbine and transmit certain, meaningful wavelengths of optical energy to determine temperature. More specifically, the present invention includes improvements in the relationship of the various elements of the optical system to each other and to the flame. A lens is positioned so that it collects infrared (IR) radiation from that portion of the flame nearest the inlet to the turbine section. The lens focuses the IR energy on one end of an optical fiber, with a mounting structure supporting one end of the optical fiber in fixed relation to the flame. Compressed air is supplied to the mounting structure to shield the lens from combustion gases in the flame. The other end of the fiber is positioned to direct a beam of IR energy onto a plurality of detectors positioned in a second mounting structure spaced from the turbine. A spectral separation mechanism is provided before the detectors to separate the incident IR radiation into a plurality of narrow-range IR frequencies. An optical chopping mechanism is provided for interrupting the IR beam (at a predetermined frequency) before the beam reaches the spectral separating mechanism. In this way, the detector receives a chopped, narrow-range IR signal. The signals are converted to appropriate electrical signals, processed to determine optical energy, and preferably compared to a predefined look-up table to determine a temperature value for a given set of detected optical energy signals.
It should be emphasized that the disclosure in this application includes xe2x80x9cbest modexe2x80x9d descriptions of preferred related technologies (e.g., temperature calculation via a look-up table) which are not part of the instant claimed invention. This disclosure is amplified for purposes of completeness.
Accordingly, the present invention provides a system and method for determining combustion temperature using infrared emissions. The present invention includes a sensor, a signal conditioning stage and a temperature determining stage to provide temperature measurement at a turbine inlet location.
In the present invention, an optical system is focused on the flame as the temperature to be measured. As mentioned above, it is desired to measure the flame temperature when the combustion process is essentially complete, i.e., the gaseous products of combustion contain stable compounds of H2O and CO2. For this purpose, the IR radiation for that portion of the flame closest to the turbine inlet is measured. The resultant optical signal is focused on one end of a fiber optic cable and the other end of the fiber optic cable emits light into an optical detection system. This optical detection system includes an optical chopper, after which the optically chopped signal impinges on a number of separate detectors which convert the optical signal into an electric current. Each of the optical sensors is preferably provided with a selective filter which passes only a very limited, discrete range (i.e., narrow band filter) of infrared wavelength. In the preferred embodiment, four wavelength filters are used: one to pass wavelengths of radiation specifically emitted by CO2, one to pass wavelengths specifically emitted by H2O, one to pass a correlated wavelength of CO2 and H2O, and one to pass a background radiation wavelength. The resultant composite signal is then processed to obtain a stable optical energy signal at each of the selected wavelengths. Preferably, the signal processing includes programmable gain amplifiers and digital to analog circuits for preparation of the signals for computer calculations.
The IR signals must be fed to an optical detector which is subject to careful temperature control so that temperature effects of the detector can be eliminated in so far as is technically feasible. The optical chopper causes a zero optical signal to be available at a given chopping rate (such as 65 Hz) as well as the regular optical signal. Since only the difference between the two signals is used, any DC slow drift is eliminated. In this case, each channels"" programmable gain amplifier is controlled by the computer and the signal processing system so that the signal remains in the middle A/D range where accuracy is best. The hardware also includes the use of the digital to analog converters to generate an offset to the signal to assist in further keeping the A/D conversion accurate.
A calibration is also performed. The purpose of calibrating the instrument is to account for component variations from sensor to sensor. Calibration consists of converting an electrical (voltage) signal from each detector element, to an optical (radiation) signal which is used in the software program to determine temperature. A blackbody radiation source is used for this purpose. Since the amount of radiation exiting a blackbody source is well known, there is a direct relation to the detector response. A standard instrument blackbody with emissivity  greater than 0.99 has a very well defined spectral emission as a function of its temperature. Optical radiation, at different blackbody temperatures, transmits through the entire optical system, and the voltage response from each of the four detector elements is measured. The detector output as a function of uwatts/steradian-cm is then calculated for each blackbody temperature. These data yield a graph to convert detector reading to the radiation intensity valued that are necessary for flame temperature back calculation.
The temperature calculation is performed by using a multidimensional look-up-table (LUT). The LUT is created by the following four steps. (1) A stochastic simulation is carried out to mimic the CO2 and H2O concentrations and temperatures over a broad range of values, and over the path length present in the turbine. The CO2 concentrations vary from 0.005 to 0.08 mole fraction and H2O varies from 0.005 to 0.16 mole fraction. The temperature is varied over the range of interest from 500xc2x0 C. to 1400xc2x0 C. (2) The radiation intensities leaving these simulated paths are calculated using a narrow band model such as RADCAL. (3) Preferably, the resultant intensities are first sorted into a four dimensional table, with the radiation at each of the three wavelengths arranged in three columns. The temperature corresponding to the three intensity values are stored in a fourth column, in the four-dimensional LUT. (4) The sorted values are then averaged to provide a convenient number of intensities (typically 8 to 50) along the three dimensions, with temperature forming the fourth dimension. This table forms the LUT.
After the LUT is obtained, it is stored into memory. During operation, one of the intensity values is chosen as a background radiation channel and is used to correct the intensities of the other 3 wavelengths, and the corrected intensities at these three wavelengths are used to find the temperature using a sequential search routine. This search is very fast, since the LUT has been sorted in an ascending (or descending) order. To improve speed, equi-spaced intensities (or the logarithm of the intensities) with indexing can also be used.
The details of the various aspects of the system are described below in more detail hereinafter, and are specifically claimed in the copending application of Sivathanu, application Ser. No. 09/232,424 filed on even date herewith.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use, the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.
Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein: